Speech recognition is a powerful tool for users to provide input to and interface with a computer. Because speech does not require the operation of cumbersome input tools such as a keyboard and pointing devices, it is the most convenient manner for issuing commands and instructions, as well as transforming fleeting thoughts and concepts into concrete expressions or words. This is an especially important input mechanism if the user is incapable of operating typical input tools because of impairment or inconvenience. In particular, users who are operating a moving vehicle can more safely use speech recognition to dial calls, check email messages, look up addresses and routes, dictate messages, etc.
Some elementary speech recognition systems are capable of recognizing only a predetermined set of discrete words spoken in isolation, such as a set of commands or instructions used to operate a machine. Other speech recognition systems are able to identify and recognize particular words uttered in a continuous stream of words. Another class of speech recognition systems is capable of recognizing continuous speech that follows predetermined grammatical constraints. The most complex application of speech recognition is the recognition of all the words in continuous and spontaneous speech useful for transcribing dictation applications such as for dictating medical reports or legal documents. Such systems have a very large vocabulary and can be speaker-independent so that mandatory speaker training and enrollment is not necessary.
Conventional speech recognition systems operate on recognizing phonemes, the smallest basic sound units that words are composed of, rather than words. The phonemes are then linked together to form words. The phoneme-based speech recognition is preferred in the prior art because very large amounts of random access memory is required to match words to sample words in the library, making it impracticable.